1. Field of the Invention
The present invention relates to novel members of the Tumor Necrosis Factor (TNF) receptor family. More specifically, isolated nucleic acid molecules are provided encoding a human TNF receptor-related protein, referred to herein as the TR2 receptor of FIG. 1A-1B, having considerable homology to murine CD40. Two different TR2 splice variants, referred to as TR2-SV1 and TR2-SV2, are also provided. TR2 polypeptides are also provided with homology to human type 2 TNF receptor (TNF-RII). Further provided are vectors, host cells and recombinant methods for producing the same. The invention also relates to both the inhibition and enhancement of functional activities of TR2 receptor polypeptides and diagnostic methods for detecting TR2 receptor gene expression.
2. Related Art
Human tumor necrosis factors α (TNF-α) and β (TNF-β or lymphotoxin) are related members of a broad class of polypeptide mediators, which includes the interferons, interleukins and growth factors, collectively called cytokines (Beutler, B. and Cerami, A., Annu. Rev. Immunol. 7:625-655 (1989)).
Tumor necrosis factor (TNF-α and TNF-β) was originally discovered as a result of its anti-tumor activity, however, now it is recognized as a pleiotropic cytokine playing important roles in a host of biological processes and pathologies. To date, there are ten known members of the TNF-related cytokine family, TNF-α, TNF-β (lymphotoxin-α), LT-β, TRAIL and ligands for the Fas receptor, CD30, CD27, CD40, OX40 and 4-1BB receptors. These proteins have conserved C-terminal sequences and variable N-terminal sequences which are often used as membrane anchors, with the exception of TNF-β. Both TNF-α and TNF-β function as homotrimers when they bind to TNF receptors.
TNF is produced by a number of cell types, including monocytes, fibroblasts, T-cells, natural killer (NK) cells and predominately by activated macrophages. TNF-α has been reported to have a role in the rapid necrosis of tumors, immunostimulation, autoimmune disease, graft rejection, producing an anti-viral response, septic shock, cerebral malaria, cytotoxicity, protection against deleterious effects of ionizing radiation produced during a course of chemotherapy, such as denaturation of enzymes, lipid peroxidation and DNA damage (Nata et al., J. Immunol. 136(7):2483 (1987)), growth regulation, vascular endothelium effects and metabolic effects. TNF-α also triggers endothelial cells to secrete various factors, including PAI-1, IL-1, GM-CSF and IL-6 to promote cell proliferation. In addition, TNF-α up-regulates various cell adhesion molecules such as E-Selectin, ICAM-1 and VCAM-1. TNF-α and the Fas ligand have also been shown to induce programmed cell death.
TNF-β has many activities, including induction of an antiviral state and tumor necrosis, activation of polymorphonuclear leukocytes, induction of class I major histocompatibility complex antigens on endothelial cells, induction of adhesion molecules on endothelium and growth hormone stimulation (Ruddle, N. and Homer, R., Prog. Allergy 40:162-182 (1988)).
Both TNF-α and TNF-β are involved in growth regulation and interact with hemopoietic cells at several stages of differentiation, inhibiting proliferation of various types of precursor cells, and inducing proliferation of immature myelomonocytic cells. Porter, A., Tibtech 9:158-162 (1991).
Recent studies with “knockout” mice have shown that mice deficient in TNF-β production show abnormal development of the peripheral lymphoid organs and morphological changes in spleen architecture (reviewed in Aggarwal et al., Eur Cytokine Netw. 7(2):93-124 (1996)). With respect to the lymphoid organs, the popliteal, inguinal, para-aortic, mesenteric, axillary and cervical lymph nodes failed to develop in TNF-β−/− mice. In addition, peripheral blood from TNF-β−/− mice contained a three fold reduction in white blood cells as compared to normal mice. Peripheral blood from TNF-β−/− mice, however, contained four fold more B cells as compared to their normal counterparts. Further, TNF-β, in contrast to TNF-α has been shown to induce proliferation of EBV-infected B cells. These results indicate that TNF-β is involved in lymphocyte development.
The first step in the induction of the various cellular responses mediated by TNF-α or TNF-β is their binding to specific cell surface or soluble receptors. Two distinct TNF receptors of approximately 55-KDa (TNF-RI) and 75-KDa (TNF-RII) have been identified (Hohman et al., J. Biol. Chem., 264:14927-14934 (1989)), and human and mouse cDNAs corresponding to both receptor types have been isolated and characterized (Loetscher et al., Cell, 61:351 (1990)). Both TNF-Rs share the typical structure of cell surface receptors including extracellular, transmembrane and intracellular regions.
These molecules exist not only in cell bound forms, but also in soluble forms, consisting of the cleaved extra-cellular domains of the intact receptors (Nophar et al., EMBO Journal, 9(10):3269-76 (1990)) and otherwise intact receptors wherein the transmembrane domain is lacking. The extracellular domains of TNF-RI and TNF-RII share 28% identity and are characterized by four repeated cysteine-rich motifs with significant intersubunit sequence homology. The majority of cell types and tissues appear to express both TNF receptors and both receptors are active in signal transduction, however, they are able to mediate distinct cellular responses. Further, TNF-RII was shown to exclusively mediate human T-cell proliferation by TNF as shown in PCT WO 94/09137.
TNF-RI dependent responses include accumulation of C-FOS, IL-6, and manganese superoxide dismutase mRNA, prostaglandin E2 synthesis, IL-2 receptor and MHC class I and II cell surface antigen expression, growth inhibition, and cytotoxicity. TNF-RI also triggers second messenger systems such as phospholipase A2, protein kinase C, phosphatidylcholine-specific phospholipase C and sphingomyelinase (Pfefferk et al., Cell, 73:457-467 (1993)).
Several interferons and other agents have been shown to regulate the expression of TNF receptors. Retinoic acid, for example, has been shown to induce the production of TNF receptors in some cells type while down regulating production in other cells. In addition, TNF-α has been shown to affect the localization of both types of receptor. TNF-α induces internalization of TNF-RI and secretion of TNF-RII (reviewed in Aggarwal et al., supra). Thus, the production and localization of both TNF-Rs are regulated by a variety of agents.
Both the yeast two hybrid system and co-precipitation and purification have been used to identify ligands which associate with both types of the TNF-Rs (reviewed in Aggarwal et al., supra and Vandenabeele et al., Trends in Cell Biol. 5:392-399 (1995)). Several proteins have been identified which interact with the cytoplasmic domain of a murine TNF-R. Two of these proteins appear to be related to the baculovirus inhibitor of apoptosis, suggesting a direct role for TNF-R in the regulation of programmed cell death.